In the production of paper pulp from comminuted cellulosic fibrous material, such as wood chips, by using a continuous digester, it has been found that the quality of the chemical pulp produced is significantly affected by how much mechanical action the chips are subjected to during the treatment process. Mechanical handling of, and mechanical action on, the chips damages them, and reduces their size. This is especially so when high temperature or pressure conditions exist, or chemicals are present, as the chips are being acted upon by a mechanical device. For example, a conveying screw that slants at 30.degree. to the horizontal, and such as utilized conventionally at the top of some vapor phase/liquid phase digesters, increases the amount of pinchips (which is an indicator of material damage) greater than 1.4%, while a horizontal conveying screw (if there is no significant adverse pressure or temperature condition) would increase the number of pinchips by less than 0.4%. This increase in damage to the material is just a visible increase in damage, and in fact there is additional damage to the chips that is not readily noticeable.
In the production of chemical pulp from continuous digesters, the trend----since the beginning of continuous cooking----has been toward larger and larger vessels. As a matter of fact, the size of some conventional digesters has increased to the point where uniform treatment is extremely difficult to achieve. This is so since every time the digester diameter is doubled, the distance for displacement of liquid is doubled, the cross-sectional area is quadrupled, and the volume is increased 8 times. This has resulted in the utilization of "cheater flows" in some large digesters, and even with "cheater flows" desired results are not necessarily achieved since the wash efficiency is significantly decreased and quality and yield may still be unacceptable.
The most common conventional digester systems utilized are the hydraulic digester, steam phase digester, two vessel steam phase digester, two vessel hydraulic digester, and two vessel hydraulic with modified continuous cooking. All of these systems are subject to the quality problems mentioned above, and are limited by the size difficulties also discussed above. While the hydraulic digester often has good heat economy and minimizes damage to the chips since it has just one top separator and a cold blow (which neutralizes the damaging effect of the digester outlet), it is sensitive to poor furnish which provokes "hang-ups", heater flow is less than desirable, and the dependability and quality are questionable past a capacity of 1,000 tons per day.
Conventional steam phase digesters are typically easy to operate, and less dependent on furnish hang-ups than hydraulic digesters, but there is reduced heat economy due to the use of direct steam, and there can be quality problems associated with the way that the chips are steamed in a conventional horizontal steaming vessel, and there is a risk of hammering. The two vessel steam please digester systems reduce tailings and therefore increases the uniformity of the product, and possibly improves the heat balance, but does so at a loss of strength and quality since the chips are subjected to more mechanical action. Two vessel hydraulic systems have advantages over conventional hydraulic digesters, however heating is effected by mixing rather than displacement so that the chips are heated to about 4.degree.-7.degree. C. above the final cooking temperature, resulting in increased pressure, and undesirable action on the chips, and undesirable complications for the system. Two vessel hydraulic systems with modified continuous cooking allow the production to be increased so that 1,500 tons per day capacity is not unusual, however when the chips are mechanically acted upon this often is in the presence of white liquor, or higher concentrations of white liquor than in other systems, resulting in increased chip degradation.
According to the present invention, a continuous cooking apparatus and method are provided that address the quality drawbacks, size limitations, and energy efficiency problems that are inherent in modern continuous systems. According to the present invention, quality is enhanced by reducing the mechanical action on the chips, especially under temperature, pressure, and consistency conditions which result in the most severe degradation. Quality is further addressed by enhancing the uniformity of the treatment. Size limitations are also addressed in some embodiments by effectively minimizing the displacement distances. Energy efficiency is also dealt with by minimizing heat loss. The desirable results achievable according to the present invention can be achieved in a relatively simple manner, with only minor changes to existing configurations, utilizing only conventional components, and at a reasonable price.
According to one aspect of the present invention, an apparatus for the continuous cooking of wood chips to produce paper pulp is provided which includes a generally vertically disposed impregnation vessel having a top and a bottom, and a generally vertically disposed digester having a top and a bottom with an inlet at the top. Means are provided for feeding a liquid slurry of wood chips to the bottom of the impregnation vessel, and at the bottom of the impregnation vessel a mechanical separating means for separating the chips from some of the liquid (so as to decrease the liquid to material ratio of the slurry) is provided. Such mechanical separating means preferably takes the form of a feeder screw rotatable in a perforated cylinder (i.e. a conventional "top separator"). The top of the impregnation vessel is adjacent the top of the digester, and means are provided for transferring the material from the top of the impregnation vessel to the top of the digesting vessel with a minimum of mechanical action on the material. Typically, a small rotating distributor will be provided, but it is also possible to effect transfer utilizing only means for establishing fluid flows. Extraction screens are disposed in the digester, which may be "live" screens, and the pulp is discharged from the bottom of the digester, as is conventional.
According to the preferred embodiment, the impregnation vessel is located within, and generally concentric with, the digester. Not only does this increase pulp quality by providing the bulk of the mechanical action on the chips when they are the coolest, under the least pressure, and at a relatively high liquor to material ratio, but it addresses the size and energy efficiency problems at the same time. By locating the impregnation vessel within the digester, energy efficiency is enhanced, and also smaller volumes for displacement are provided since the material moving in the digester moves in a ring shape, the center of the digester being occupied by the impregnation vessel. Treatment liquid (typically white liquor) is added in the digester at various points along its length by conduits which are attached to the exterior of the impregnation vessel, and terminate at different heights along its length, uniformly adding the liquor at those points. The apparatus may comprise either a steam digester or an hydraulic digester; in the former, high pressure steam is preferably added at the top of the digester, and there is no need to use a conventional horizontal steaming vessel prior to the impregnation vessel.
The impregnation vessel also may be separate from the digester, and it may be insulated (e.g. double walled) in order to improve energy efficiency.
According to the present invention there also is provided a method of continuously digesting comminuted cellulosic fibrous material to produce paper pulp by practicing----substantially sequentially and continuously----the following steps: (a) Feeding a liquid slurry of comminuted cellulosic fibrous material at a liquid to material ratio of about 20-25/1 to the bottom of the impregnation vessel. (b) Separating some of the liquid from the slurry at the bottom of the impregnation vessel to provide a liquid to material ratio in the impregnation vessel of about 5-8/1. (c) Impregnating the material with liquid as it flows upwardly in the impregnation vessel from the bottom to the top thereof. (d) Moving the material from the top of the impregnation vessel to the top of the digesting vessel with a minimum of mechanical action on the material. (e) Digesting the material in the digesting vessel, utilizing digesting liquid, at a liquid to material ratio of about 3-5/1; and (f) withdrawing pulp from the bottom of the digesting vessel. The typical residence time in the impregnation vessel is about 20 to 30 minutes.
The invention also contemplates a system, in general, for pulp treatment. According to this aspect of the invention there is provided: A first generally vertically disposed impregnation vessel having a top and a bottom. A second generally vertically disposed vessel having a top and a bottom. The tops of the two vessels being located adjacent each other, and interconnected so that slurry may flow from one to the other, with the first vessel located within, and generally concentric with, the second vessel. A liquid/material separating means located at the bottom of the first vessel, comprising a feeder screw mounted within a perforated screen cylinder, and rotatable with respect to the cylinder; and, screen means located adjacent the top of the first vessel for removing some liquid from the slurry at the top of the first vessel.
It is the primary object of the present invention to enhance the quality of chemical pulps produced by continuous digestion. This and other objects of the invention will become clear from the detailed description of the invention, and from the appended claims.